igneous
Polished black obsidian
Volcanic glass; primarily silica-rich rhyolitic composition (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectUsually black, glossy, and noncrystalline, with conchoidal fracture; Mohs hardness about 5–5.5, vitreous luster, no cleavage, and specific gravity roughly 2.3–2.6. The smoothly rounded, polished form obscures its natural fracture surfaces.
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Physical properties
Usually black, glossy, and noncrystalline, with conchoidal fracture; Mohs hardness about 5–5.5, vitreous luster, no cleavage, and specific gravity roughly 2.3–2.6. The smoothly rounded, polished form obscures its natural fracture surfaces.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, generally during Quaternary to recent volcanic eruptions. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for carvings, beads, cabochons, decorative objects, and historically sharp cutting tools and arrowheads; this specimen appears shaped and highly polished for jewelry or collecting.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. It can display snowflake, mahogany, rainbow, or sheen effects from inclusions and microscopic structures.
Field identification & locations
Identify by its uniform glassy black appearance, very smooth polish, light weight relative to metallic minerals, and conchoidal fracture; it should not show metallic streak or crystal faces. Avoid confusing it with black glass, jet, or tourmaline.
